Re: [PATCH] drm/panel: chipwealth-ch13726a: Fix dark grays rendering green at 120 Hz

[Matthew Schwartz <matthew.schwartz@linux.dev>]

On 10/27/26 9:57 PM, Aaron Kling wrote:
> On Tue, Oct 27, 2026 at 9:02 PM Matthew Schwartz
> <matthew.schwartz@linux.dev> wrote:
>>
>> The init sequence writes 0xf0 0x50 followed by 0xb9 0x00. The panel
>> configuration in the Thor's stock Android firmware (decompiled from its
>> devicetree overlay, dtbo.img) has two runtime command sequences that
>> differ only in the 0xb9 value, both "display off; f0 50; b9 xx; display
>> on":
>>
>>   qcom,mdss-dsi-ram-command:        b9 11
>>   qcom,mdss-dsi-bypass-ram-command: b9 00
>>
>> With 0x00, in the 120 Hz mode, a full-screen gray ramp from black to
>> white, shown with identity color management, renders black as green and
>> the darkest grays as bright green, both at maximum and at reduced
>> brightness, and the tint is more noticeable at lower brightness. The
>> 60 Hz mode renders it correctly. With 0x11 the ramp renders correctly in
>> both modes and the panel dims without a color shift.
>>
>> Use 0x11, the value from the vendor's ram-command sequence, instead.
>>
>> Fixes: 3ee01b8647b5 ("drm/panel: Add panel driver for ChipWealth CH13726A based panels")
>> Reported-by: Aaron Kling <webgeek1234@gmail.com>
>> Link: https://lore.kernel.org/all/20260727-ch13726a-washout-v1-1-bf869ddfc5cf@gmail.com/
>> Signed-off-by: Matthew Schwartz <matthew.schwartz@linux.dev>
>> ---
>> Aaron, this should be the 120 Hz issue from your washout patch; could
>> you give it a try on your Thor? I found the issue here separately and
>> saw your patch as I was prepping for submission, and I bet I was seeing
>> the same bug you originally saw.
> 
> This is something we tried before. And best we can tell, enabling gram
> limits frame rate to 60hz on this panel. Framerate tests such as the
> ufo test were reporting 60 when set up like this. Can you verify the
> refresh rate on your device?

Hmm, yeah, it was a bit hard to tell until instrumenting it but it
seems to use a panel refresh rate of ~63 Hz. I do wonder if this panel
is just not meant to be driven at 120 Hz though, since it seems like a
very poor choice to expose as a mode with color issues like this.

It might make more sense to just not expose it if it isn't fixable.

- Matt

> 
> Aaron
> 
>>  drivers/gpu/drm/panel/panel-chipwealth-ch13726a.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/panel/panel-chipwealth-ch13726a.c b/drivers/gpu/drm/panel/panel-chipwealth-ch13726a.c
>> index 562dc573528d..0832e49b77a9 100644
>> --- a/drivers/gpu/drm/panel/panel-chipwealth-ch13726a.c
>> +++ b/drivers/gpu/drm/panel/panel-chipwealth-ch13726a.c
>> @@ -65,7 +65,8 @@ static int ch13726a_on(struct ch13726a_panel *ctx)
>>         ctx->dsi->mode_flags |= MIPI_DSI_MODE_LPM;
>>
>>         mipi_dsi_generic_write_seq_multi(&dsi_ctx, 0xf0, 0x50);
>> -       mipi_dsi_generic_write_seq_multi(&dsi_ctx, 0xb9, 0x00);
>> +       /* 0x00 (the vendor "bypass ram" sequence) renders near-black as green */
>> +       mipi_dsi_generic_write_seq_multi(&dsi_ctx, 0xb9, 0x11);
>>
>>         mipi_dsi_dcs_exit_sleep_mode_multi(&dsi_ctx);
>>
>>
>> base-commit: 8ef66514d7b993c9fb96a5886f001e3556610c1c
>> --
>> 2.56.0
>>